Generally, a hybrid electric vehicle is a vehicle that uses two or more types of driving sources, such as an internal engine system and an electric motor, and various kinds of power transmission systems including the internal engine system and the electric motor.
A representative power transmission system of the hybrid electric vehicle includes a parallel type system and a serial type system. The serial type system is configured such that the internal engine system and the electric motor are serially coupled. Thus, although the serial type system has advantageously relatively simple structure and control logic to those of the parallel type system, it is disadvantageous in terms of energy conversion efficiency, since mechanical energy from the engine system is stored in a battery module in the form of electric power, which is then used to activate the electric motor to drive the vehicle. Conversely, although the parallel type system has disadvantageously relatively complex structure and control logic to those of the serial type system, it is advantageous in terms of energy efficiency, since it can use the mechanical energy from the internal engine system and the electric energy from the battery module at the same time.
A conventional parallel type hybrid electric vehicle system uses a high voltage start generator (HSG) or a low voltage starter as an engine starter. The system employing the low voltage starter as an engine starter is advantageous in terms of packaging of an engine room and saving for system cost, but disadvantageous in securing driving safety in preparation for a failure of a motor system generating driving power, compared to the HSG employed system.
This is because a high voltage battery unit generating high voltage in the battery module of the parallel type system using the low voltage starter is charged during driving of the hybrid vehicle system by an electric motor which is only a high voltage type component in the parallel type system. Thus, when a motor system fails, the only method of charging the high voltage battery unit during driving of the hybrid vehicle is lost. In this case, the high voltage battery unit is discharged while supplying electric power to a low voltage DC-DC converter (LDC), resulting in an over-discharged state.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.